1. Field of the Invention
The present invention relates to a porous electrode, and more particularly to a porous electrode used for conductive material-filled polymer composite.
2. Description of the Prior Art
Conductive material-filled polymer composites are well-known and include a polymer substrate and conductive particles that are filled in the polymer substrate. Since they have good electrical physical properties and a wide range of processability, conductive material-filled polymer composites are often used as conductive material for various resistors and positive temperatuve coefficient (PTC) passive devices. The conductive particles filled include carbon black, nickel powders, silver powders, and graphite. Due to limitations on conductivity, wear resistance, and soldering problems, a metal electrode should be bonded onto the surface of the conductive material-filled polymer composite; thus, it is more convenient to undergo the subsequent processing on the passive device.
FIG. 1 shows the bonding situation of two conventional metal electrode layers and a conductive material-filled polymer composite. The conductive material-filled polymer composite 10 is sandwiched between two metal electrode 11 layers. The surface 12 of the metal electrode 11 contacting the conductive material-filled polymer composite 10 is smooth.
When such a smooth metal electrode is bonded to the conductive material-filled polymer composite, the following problems arise. The difference of the thermal expansion coefficient between the metal electrode and the conductive material-filled polymer composite will induce poor adhesion between them. When the conductive material-filled polymer composite is utilized for various resistors and PTC passive devices, it encounters various cyclic or non-cyclic temperature changes; therefore, the adhesion requirements of the metal electrode will become even more critical. In addition, the interfacial resistance between the conductive material-filled polymer composite and the metal electrode is also a problem. Since the conductive material-filled polymer composite is composed of conductive particles and insulate polymer substrate, during the process of bonding the metal electrode to the conductive material-filled polymer composite, the insulate polymer substrate may flow and fill into the space between the metal electrode and the conductive particles. This will induce the increase of the interfacial resistance, thus affecting the electrical properties of the conductive material-filled polymer composite after the electrode is bonded.
To solve the problem that the adhesion strength between the conductive material-filled polymer composite and the metal electrode is poor, the concept of roughing the metal electrode surface is first disclosed in U.S. Pat. No. 4,689,475 (Raychem Corporation). Referring to FIG. 2, a conductive material-filled polymer composite 20 is sandwiched between two electrode layers 21. The electrode layer 21 is contacted with the conductive material-filled polymer composite layer 20 by a rough surface 22. On the rough surface 22, there are a plurality of projections 23 with a height of 0.1 .mu.m to 100 .mu.m. In this manner, since the projections 23 on the rough surface 22 are inserted into the conductive material-filled polymer composite 20, the adhesion strength between the electrode 21 and the conductive material-filled polymer composite 20 can be increased.
In U.S. Pat. No. 4,800,253 (Raychem Corporation), the concept of U.S. Pat. No. 4,689,475 is further improved by a structure shown in FIG. 3. Referring to FIG. 3, a conductive material-filled polymer composite layer 30 is sandwiched between two electrode layers 31. The electrode layer 31 is contacted with the conductive material-filled polymer composite layer 30 by a rough surface 32. On the rough surface 32, there are a plurality of macronodules 33 with a height of 0.1 .mu.m to 100 .mu.m. On the macronodule 33, there are a plurality of micronodules 34 with a height of 0.5 .mu.m to 2 .mu.m. In this manner, since the macronodules 33 and the micronodules 34 on the rough surface 32 are inserted into the conductive material-filled polymer composite 30, the adhesion strength between the electrode 31 and the conductive material-filled polymer composite 30 can be further increased.
In WO 9636057, the electrode used to be contacted with the conductive material-filled polymer composite has an open type cellular structure, which can improve the adhesion strength between the electrode and the conductive material-filled polymer composite.
In the above-mentioned two Raychem patents, the adhesion strength between the metal electrode and the conductive material-filled polymer composite is improved by making the electrode rough to cause physical insertion between them. However, during the process of bonding the metal electrode and the conductive material-filled polymer composite, the insulate polymer substrate will still flow and fill into the space between the metal electrode and the conductive particles, which increases the interfacial resistance. As to WO 9636057, the direct contact between the conductive particles of the conductive material-filled polymer composite and the electrode still needs further improvement.